1. Field of the Invention
The present invention relates to novel reflector structures in general, and in particular to antenna systems for microwave and millimeter wave electromagnetic radiation.
2. Description of the Prior Art
When a reflector assembly operates as a transmitting antenna, a radiation feed, horn, or other component associated with an electromagnetic wave transmitting device is placed on the axis of the assembly. In some systems such a component is placed at the focus of a single reflector, and projects electromagnetic waves toward the reflector. In other systems it projects waves toward a secondary reflector that re-directs them to a primary reflector. The primary reflector is a concave paraboloid; it collimates the electromagnetic waves coming from the transmitting device or secondary reflector. The secondary reflector in a Gregorian system is a coaxial concave ellipsoid. In a Newtonian system it is flat. In a Cassegrainian system it is a coaxial convex hyperboloid.
When the assembly operates as a receiving antenna, the primary reflector collects incoming electromagnetic waves. It directs them to a component such as a horn associated with an electromagnetic wave receiving device, or to a secondary reflector that redirects them to such a component.
A secondary reflector displaces the effective focus of an antenna back toward the primary reflector. This makes it possible to mount an electromagnetic wave transmitting or receiving device near the primary reflector, reducing the axial dimension of the assembly. A Cassegrainian system is the most compact configuration. However, the concave ellipsoidal secondary reflector of a Gregorian system is easier to fabricate than the convex hyperboloidal secondary reflector of a Cassegrainian system. The flat secondary reflector of a Newtonian system is the simplest. Foamed polymers are ideal materials for fabrication of such components.
Auletti (U.S. Pat. No. 4,482,513) forms microwave lenses of foam. He brings the effective dielectric constant to the desired value by mixing aluminum flakes in foam resin before pouring it into a lens mold. His invention is for refracting antennas rather than reflecting antennas.
Myer (U.S. Pat. No. 4,636,801) takes advantage of the high strength-to-weight ratio of a foamed polymer material, but does not make use of its low dielectric constant. His primary reflector is a metal layer bonded to a concave paraboloidal surface on a foam body. The foam is behind the reflector; the reflecting surface is exposed. A secondary reflector also has an exposed reflecting surface with foam behind a metal layer. The secondary reflector is supported by spider legs attached to the foam body of the primary reflector. Major portions of Myer's description and claims are devoted to the spider legs. Fabrication of the assembly requires skilled hand labor to achieve precise placement of the spider legs and secondary reflector relative to the primary reflector. After the spider legs and secondary reflector are set in place, the assembly must remain undisturbed for a period of time to allow an adhesive to form a bond between the parts.
Rothstein (U.S. Pat. No. 5,057,844) recognizes the benefit of protecting a metal antenna with a material of low dielectric constant. He sandwiches a flat strip antenna between flat pieces of polystyrene foam. The foam pieces do not shape the antenna; they merely enclose it for protection from a corrosive environment.
Knox (U.S. Pat. No. 4,188,632) shows a secondary reflector or splash plate attached to a dielectric body in front of a waveguide. This subassembly is only part of a larger system that includes a primary reflector which Knox does not show. The splash plate blocks a portion of the primary reflector; a small splash plate is desirable. The dielectric body acts as a lens to change the directions of waves reflected by the splash plate, making possible the use of a smaller splash plate. A foam with a low dielectric constant would require a larger splash plate, defeating Knox's purpose. Knox shows a rod-like extension from the dielectric body, continuing with a tapered portion. It is a long slender member deeply inserted in a tightly-fitting waveguide. Its purpose is to match the impedance from air in the waveguide to the external body with a higher dielectric constant. Care is required to avoid breaking off this member in the process of inserting it into the waveguide. This does not facilitate rapid assembly in a manufacturing operation. Regardless of the speed of assembling the waveguide/splash plate subassembly, Knox's dielectric extension does not key the location of the waveguide/splash plate subassembly relative to a primary reflector.
Iida (Japanese Patent No. 56-122,508) describes a horn/waveguide subassembly for mounting in front of a primary reflector. Iida does not show the primary reflector or mechanical keys for locating the subassembly relative to it. Iida's subassembly performs a function similar to that of Knox. Iida shows a dielectric wave director that serves as an extension of a horn. This dielectric body directs waves by internal reflection, confining them within the dielectric in transit from the metal horn to a convex subreflector. The convex subreflector changes the wavefront directions to enable reflected waves to pass through the dielectric/air interface at angles away from the critical angle for total internal reflection. Total internal reflection requires a dielectric constant greater than that of air, so a foam dielectric would not serve Iida's purpose.
Jones (U.S. Pat. No. 3,611,396) shows a foam body in the form of a horn with corrugated walls and a flat septum between top and bottom sections. The surfaces are plated with metal by a complex process, the subject of another patent application. The corrugated surfaces are not compatible with rapid attachment of layers of low-cost electrically conducting materials such as foils or wire fabrics.
Lier et al. (U.S. Pat. No. 4,783,665) describe dielectric horns that serve mainly to support metal grid structures in front of metal horns. Such a modified horn functions in a manner similar to that of a corrugated horn.
Berg (Swedish Patent No. 170,502) shows foam between the concave primary reflector and the convex secondary reflector of a Cassegrainian antenna. The foam does not extend into a horn at the center of the primary reflector. The horn is attached to the primary reflector. The reflectors are pre-formed metal shells. Berg does not disclose a fabrication process, but the assembly shown in his single drawing could be fabricated by foaming in place, holding the primary and secondary reflectors in their required positions relative to each other and allowing a foaming resin to expand between them. In this process the foam is shaped by the pre-formed reflector shells. Berg does not teach the lamination of metal foils, electrically conducting polymer films, wire screens, or electrically conducting fabrics on a pre-formed foam body.